1. Field of the Invention
The present invention relates to a cooling water circuit system of a water-cooled engine (hereinafter referred to as "engine").
Conventionally, there has been proposed an apparatus in which a heat-accumulating tank for thermally insulating and reserving cooling water for cooling the engine is disposed in a cooling water circuit and an acceleration of a warm-up operation of the engine and a quick heating operation are performed by using the high-temperature cooling water accumulated in the heat-accumulating tank when a temperature of the cooling water is low after the engine has been just started.
However, in the above-described apparatus, since the acceleration of the warm-up operation of the engine and the quick heating operation are performed by using the high-temperature cooling water accumulated in the heat-accumulating tank, the heat-insulating capacity of the heat-accumulating tank has a great influence on the effects of the acceleration of the warm-up of the engine and the quick heating operation.
The inventors have experimentally produced a cooling water circuit including various kinds of heat-accumulating tank, and found out that the following points relate to the improvements of the heat-insulating capacity of the heat-accumulating tank greatly.
That is, to the heat-accumulating tank, there are connected an inflow cooling water passage for introducing the cooling water discharged from the engine to the heat-accumulating tank and an outflow cooling water passage for introducing the cooling water discharged from the heat-accumulating tank to the engine.
Therefore, when the other portions (e.g., a pipe connected to the heat-accumulating tank) than the heat-accumulating tank is communicated with the heat-accumulating tank through both passages while the engine stops, the cooling water in the other portions than the heat-accumulating tank and the cooling water in the heat-accumulating tank may be mixed by natural convection.
Further, because the other portions than the heat-accumulating tank do not have a heat insulating structure, the cooling water in the other portions than the heat-accumulating tank may be cooled earlier than the cooling water in the heat-accumulating tank. Therefore, a temperature difference between the cooling water in the other portions than the heat-accumulating tank and the cooling water in the heat-accumulating tank becomes larger, with the result that the mixture by the natural convection may be accelerated. Further, the heat-insulating capacity of the heat-accumulating tank may be deteriorated.